The present invention relates to a magnetic device. In particular, the present invention relates to a switchable permanent magnet device.
Permanent magnets consist of a ferromagnetic material that has been suitably magnetized. Permanent magnets provide a magnetic field without requiring an electric current. Permanent magnets are attracted to and can be firmly affixed ferromagnetic materials. However, permanent magnets suffer from the drawback that their energy output is fixed and they cannot be employed if a variation in magnetic field strength is required.
Due to the above-mentioned drawback with permanent magnets, it has been customary to control magnetic fields by using electric current or electromagnets. The strength and orientation of the magnetic fields of electromagnets can be controlled successfully by controlling the strength and direction of the electric current. However, this introduces the necessity of having to provide a constantly available source of electric current and associated wiring. This can cause complexity and potential hazards.
A number of attempts have also been made to arrange permanent magnets to enable variations to be achieved in magnetic field strength. A number of different approaches have been tried to produce practical solutions. However, these approaches resulted in bulky constructions, complicated constructions, or devices that were expensive to manufacture. The prior art devices also paid little attention to achieving a good holding force between the devices and substrates under conditions where an incomplete external magnetic circuit existed, for example, where there were gaps between the magnet and the keeper, or where the keeper or substrate had a low permeability, such as thin ferrous sheet metal. To achieve good holding forces in the prior art devices, it was necessary that perfect contact between the magnet and the substrate be achieved and that the substrate be of substantial volume to enable a sufficient magnetic flux to be established. This, of course, could be difficult to achieve in practice.
U.S. Pat. No. 3,121,193 (Engelstead) describes a permanent magnet-type work holding device especially a chuck that is adapted for use in turning operations. The device of Engelstead consists of arrays of permanent magnets, which magnets are of generally rectangular configuration. In order to adjust the field strength an array of pole pieces was moved away from and out of alignment with another array of pole pieces.
U.S. Pat. No. 4,241,791 (Yanagisawa) describes a magnetic base which is switchable between an engaged position and releaseable from a substance by an operation of a single rotating permanent magnet disposed notably in a magnetic circuit block. The rotary magnet in Yanagisawa is rotated by 90xc2x0 to switch between the exited and the unexcited states. This device, relies upon using passive external magnetic members of substantial mass (12 and 14 of FIG. 1 of Yanagisawa) having a rotatable magnet (20) positioned therein.
United Kingdom patent application no 2130797 relates to a permanent magnet chuck capable of switching from an excited state to an unexcited state by changing the relative positional relationship between fixed permanent magnets and movable permanent magnets. The movable permanent magnets move laterally with respect to the fixed magnets in order to vary the magnetic field.
It is an object of the present invention to provide a switchable magnetic device having improved properties.
In a first aspect, the present invention provides a switchable magnetic device comprising a housing, a first permanent magnet, a second permanent magnet, said first and second permanent magnets being essentially cylindrically shaped, said first and second permanent magnets being diametrically polarised, said first and second permanent magnets being mounted within the housing such that the first and second permanent magnets are rotatable relative to each other, and actuation means for causing relative rotation of the first and second permanent magnets, wherein said device presents a relatively strong external magnetic field when said first and second permanent magnets are positioned relative to each other such that a north and south poles of the first magnet are in substantial alignment with respective north and south poles of the second magnet, and the device presents a relatively weak external magnetic field when the first and second magnets are positioned relative to each other such that the north pole of the first magnet is in substantial alignment with the south pole of the second magnet and vice versa.
Preferable, the first and second magnets are substantially disc-shaped.
Preferably, the first magnet and the second magnet are mounted in the housing such that a face of the first magnet is opposed to a face of the second magnet. Most preferably, one magnet is mounted above the other magnet.
It is preferred that one magnet is fixedly mounted in the housing and that the other magnet is able to rotate in the housing. However, it will be appreciated that relative rotation between the magnets can also result from both magnets being rotatable. Accordingly, the present invention also encompasses devices in which both magnets can rotate.
The housing may comprise a pair of passive ferro-magnetic elements which are magnetically separated, for example, by an air gap or by magnetically high reluctance material(s). The housing most suitably defined in chamber in which the first and second magnets are positioned. The chamber may have open ends or, more preferably, is a closed chamber. Chamber closing members are suitably used to close the chamber.
In another embodiment, the housing is made as a unitary construction or from a single piece of material. In this embodiment, it is preferred that there be two portions of the housing having reduced cross sectional area such that the housing acts as two passive poles. Alternatively, portions of the housing may be treated such that the portions become non-magnetic to thereby result in the housing acting as two passive poles.
The housing is preferably made from a material having a low magnetic reluctance. Soft steel, iron or permalloys are examples of suitable materials for the housing.
The housing most preferably comprises a pair of passive poles. The strength of the external magnetic field can be maximised by shaping the pair of passive poles such that they reflect the magnetic field strength around the perimeter of the first and second permanent magnets.
The first and second magnets are preferably substantially disc-shaped. Most preferably, the first and second magnets are essentially cylindrical in shape and the height of the cylinder is less than the diameter of the cylinder. It will be appreciated that the first and second magnets need not be exactly cylindrical and that slight variations from a circular cross-section also fall within the scope of the present invention. The height of the magnets may vary over a wide range, and the ratio of diameter to height may also vary over a wide range.
The first and second magnets are also diametrically magnetised. By that, it is meant that the north pole region is separated by a diameter of the cylindrical surface of the magnet from the south pole region. The north pole region and south pole region both exist on the upper and lower substantially circular faces of the magnet and extend through the length or height of the magnet.
The first and second magnets may be essentially identical to each other.
Alternatively, the first and second magnets may have different magnetic properties. The magnets may have the same or different physical dimensions to each other. Varying the magnetic properties or the physical dimensions of the first and second magnets can be used in some embodiments of the invention to vary magnetic switching properties.
As mentioned earlier, it is preferred that one magnet is fixed in the housing and one magnet can rotate in the housing. It is especially preferred that the rotatable magnet can rotate about the centre point of its essentially circular faces. In this fashion, the requirement for large clearances between the inner walls of the housing and the rotatable magnet are avoided. In this embodiment, the shape of the body of rotation of the rotatable magnet is the same shape as the magnet itself (ie substantially circular in top or plan view) and thus the magnet can rotate and yet retain its positioning relative to the walls of the housing.
The actuation means for causing relative rotation of the magnets preferably comprises a handle or knob being in connection with one of the magnets. The handle or knob may be connected to the one magnet by one or more intermediate members.
The handle or knob may be rotated manually, electrically, pneumatically, hydraulically, by the action of expansion of a bimetallic strip, or indeed by any other suitable method
The handle may comprise a torque sensitive lever that does not allow a torque in excess of a predetermined amount to be applied to the one magnet. In this regard, when there is no external load applied to the device, it is difficult to switch device between active and inactive states. Use of a torque sensitive lever as part of the actuation means can result in it being impossible to switch the device between the active and inactive states if no sufficient external magnetic circuit is present.
As mentioned above, the housing preferably includes two passive poles, and it is also preferred that one magnet is fixed in the housing. In this particularly preferred embodiment, the one magnet is fixed in place such that the pole pieces are permanently energised.
The permanent magnets in the first aspect of the present invention may be of any suitable type. The most preferred at present are rare earth magnets because they can have a strong magnetic field. Such magnets also have a high coercivity, which means that they resist becoming demagnetised. It is envisaged that permanent magnet technology will continue to develop and it is likely that more powerful magnets will become available in the future that will be suitable for use in the present invention.
The first and second magnets may be essentially identical to each other. Alternatively, the first and second magnets may have different magnetic properties. The magnets may have the same or different physical dimensions to each other. Varying the magnetic properties or the physical dimensions of the first and second magnets can be used in some embodiments of the invention to vary magnetic switching properties.
The present invention may be used as a switchable magnetic holding device. For example, the device may be used to clasp onto surfaces, especially metal surfaces. In some embodiments, the surface to which the device is affixed is located adjacent or underneath a lower surface of the lower magnet. In other embodiments, the surface to which the device is affixed is located adjacent or beside the side surfaces of the magnets.
Alternatively, the device may be used in applications where mainly use of magnetic field is required or desirable. For example, triggering magnetic sensors, for example, in mines, particle deflection, etc.
In a second aspect, the present invention provides a switchable magnetic device of the kind including a housing, a first permanent magnet, a second permanent magnet, the first and second permanent magnets being mounted within the housing such that the first and second magnets are rotatable relative to each other, and actuation means for causing relative rotation of the first and second magnets, wherein in a xe2x80x9cfully offxe2x80x9d position the north pole of the first magnet is in substantial alignment with the south pole of the second magnet and vice versa such that the device presents a relatively weak external magnetic field and relative rotation of the magnets by 180xc2x0 moves the magnet to a xe2x80x9cfully onxe2x80x9d position in which the north pole of the first magnet is positioned in alignment with the north pole of the second magnet such that the device presents a relatively strong external magnetic field, characterised in that a biasing means is operatively associated with said actuation means to bias said magnets away from the fully off position and wherein placing said device on or in close proximity to a ferromagnetic material causes the first and second magnets to switch to a relative orientation such that the device presents a relatively strong external magnetic field.
Preferably said relative orientation comprises the fully on position.
The device of the first aspect of the present invention provides a switchable magnetic device that can automatically switch to an xe2x80x9conxe2x80x9d position in which the device presents a relative strong external magnetic field when the device is positioned on or in close proximity to a ferromagnetic material.
In a third aspect, the present invention provides a switchable magnetic device of the kind including a housing, a first permanent magnet, a second permanent magnet, the first and second permanent magnets being mounted within the housing such that the first and second magnets are rotatable relative to each other, and actuation means for causing relative rotation of the first and second magnets wherein in a xe2x80x9cfully offxe2x80x9d position the north pole of the first magnet is in substantial alignment with the south pole of the second magnet and vice versa such that the device presents a relatively weak external magnetic field and relative rotation of the magnets by 180xc2x0 moves the magnets to a xe2x80x9cfully onxe2x80x9d position in which the north pole of the first magnet is positioned in alignment with the north pole of the second magnet such that the device presents a relatively strong external field, characterised in that said device further includes rotation limiting means preventing said magnets from reaching the xe2x80x9cfully onxe2x80x9d position.
In a fourth aspect, the present invention provides a switchable magnetic device of the kind including a housing, a first permanent magnet, a second permanent magnet, the first and second permanent magnets being mounted within the housing such that the first and second magnets are rotatable relative to each other, and actuation means for causing relative rotation of the first and second magnets, wherein, in a xe2x80x9cfully offxe2x80x9d position the north pole of the first magnet is in substantial alignment with the south pole of the second magnet and vice versa such that the device presents a relatively weak external magnetic field and relative rotation of the magnets by 180xc2x0 moves the magnets to a xe2x80x9cfully onxe2x80x9d position in which the north pole of the first magnet is positioned in alignment with the north pole of the second magnet such that the device presents a relatively strong external magnetic field, characterised in that relative rotation of the first and second magnets is limited to less than 180xc2x0 such that said magnets are prevented from reaching the fully on position but said first and second magnets can be rotated to a relative orientation such that the north and south poles of the first magnet are closely but not completely aligned with respective north and south poles of the second magnet such that said device still presents a relatively strong external magnetic field and said device automatically switches to a state where the external magnetic field is relatively weak when said device is removed from contact or removed from close proximity to an external ferromagnetic material.
In a fifth aspect, the present invention provides a switchable magnetic device of the kind including a housing, a first permanent magnet, a second permanent magnet, the first and second permanent magnets being mounted within the housing such that the first and second magnets are rotatable relative to each other, and actuation means for causing relative rotation of the first and second magnets wherein in a xe2x80x9cfully offxe2x80x9d position the north pole of the first magnet is in substantial alignment with the south pole of the second magnet and vice versa such that the device presents a relatively weak external magnetic field and relative rotation of the magnets by 180xc2x0 moves the magnets to a xe2x80x9cfully onxe2x80x9d position in which the north pole of the first magnet is positioned in alignment with the north pole of the second magnet such that the device presents a relatively strong external field, characterised in that said device further includes a flexible portion that deforms if the actuation means is actuated in the absence of an external magnetic circuit to thereby limit relative rotation of said magnets, said flexible portion having sufficient yield strength to substantially avoid deformation during actuation of said actuation means in the presence of an external magnetic circuit.
Preferably, said flexible portion is a spring link.
Preferably, said flexible portion can be deformed to a maximum extent, after which continued actuation of the actuation means acts to cause relative rotation of the magnets.
In one embodiment of the fifth aspect of the present invention, if an external magnetic circuit is established and the device switched to the xe2x80x9conxe2x80x9d position, the device may remain in the xe2x80x9conxe2x80x9d position if the external magnetic circuit is forcibly removed. Thus, the device can act as a xe2x80x9cmemoryxe2x80x9d advisor in that its status can tell whether successful contact with an external magnetic circuit was achieved.
In a sixth aspect, the present invention provides a coupling comprising a coupling comprising a first magnetic device and a second magnetic device, the first magnetic device being a switchable magnetic device comprising a first permanent magnet and a second permanent magnet, said first and second permanent magnets being movable relative to each other and actuation means for effecting relative movement between the first and second magnets wherein said first magnetic device presents a relatively strong external magnetic field when the first and second magnets are positioned relative to each other such that north and south poles of the first magnet are in substantial alignment with respective north and south poles of the second magnet and the first magnetic device presents a relatively weak external magnetic field when the north pole of the first magnet is in substantial alignment with the south pole of the second magnet and vice versa, said first magnetic device and said second magnetic device being arranged relative to each other such that said second magnetic device attracts the first magnetic device when the first magnetic device presents a relative weak external magnetic field and said second magnetic device repels said first magnetic device when said first magnetic device presents a relatively strong external magnetic field.
The second magnetic device may be a magnet. Alternatively, the second magnetic device may be a second switchable magnetic device in accordance with any of the first to fifth aspects of the present invention.
The coupling of the sixth aspect of the present invention allows for both positive coupling and positive uncoupling. When the switchable magnetic device is in an xe2x80x9coffxe2x80x9d position, the second magnetic device attracts the switchable magnetic device and coupling is achieved. When the switchable magnetic device is in the xe2x80x9conxe2x80x9d position, the second magnetic device repels the switchable magnetic device and a positive uncoupling can occur.
In all aspects of the present invention, the device may further include a child-proof lock or child-proof catch type arrangement in the actuation means such that some extra manipulation by a user is required before the magnets can be rotated. For example, the actuation means may comprise a knob that must be pushed down before rotational force applied to the knob can be transferred to the magnet(s)). Other arrangements which will be readily apparent to the person of skill in the art will also be readily apparent.